--[[
A coroutine in Lua is a function which can be partially
run and suspended, rather like a generator in Python.

Although sometimes referred to as a way of threading,
it's not really that, as individual coroutines suspend
and resume under program control.
]]

-- This would normally be required in ... we need a
-- tab separated splitter!

function string.split(source)
  local result = { }
  local from = 1
  local delim_from, delim_to = string.find( source, "\t", from )
  while delim_from do
    result[#result+1] = string.sub( source, from , delim_from-1 )
    from = delim_to + 1
    delim_from, delim_to = string.find( source, "\t", from )
  end
  result[#result+1] = string.sub( source, from)
  return result
end

-- Define a function which will be used as a coroutine

--[[ This routine reads data lines like these

Station Name 0607 Entries & Exits 0607 Interchanges 0607 Estimated Total Footfall Indicative maximum funding /£K
London Waterloo "83,993,314" "4,226,542" "88,219,856" 250
London Victoria "66,749,335" "3,293,684" "70,043,019" 250

and returns a table of 5 fields, with 2 to 5 numeric

]]

function recordfeeder()
        -- A handler for a particular data format,
        -- with a requirement to spit out a specific
        -- number of records from the 2nd line of the
        -- input file onwards
        fh = (assert(io.open("st2.txt"),"ik"))
        inline = fh:read() -- skip over first line
        counter = 0
        while true do
                counter = counter + 1
                inline = fh:read()
                if inline == nil then break end
                if counter > 10 then break end
                print ("a nibble of input ...")
                fields = string.split(inline)
                -- Strip out commas and quotes too!
                for k=2,5 do
                        fields[k] = string.gsub(fields[k],'[", ]',"")
                end
                -- next line is the magic "suspend"
                coroutine.yield()
                end
        fields = nil
        end

--[[ And here's the code that sets up and calls the coroutine ]]

instream = coroutine.create(recordfeeder)
while coroutine.status(instream) ~= "dead" do
        coroutine.resume(instream)
        if fields then
                pcchan = string.format("%6.2f%%",(100 - fields[2]/fields[4]*100))
                lp = string.format("%-30s %s interchange",fields[1],pcchan)
                print (lp)
                end
        end

--[[ ------------- Sample Output ------------------------

[trainee@easterton u114]$ lua datafeeder
a nibble of input ...
London Waterloo 4.79% interchange
a nibble of input ...
London Victoria 4.70% interchange
a nibble of input ...
London Liverpool Street 2.96% interchange
a nibble of input ...
London Bridge 14.04% interchange
a nibble of input ...
London Charing Cross 4.84% interchange
a nibble of input ...
Clapham Junction 40.60% interchange
a nibble of input ...
London Paddington 6.33% interchange
a nibble of input ...
London Euston 7.10% interchange
a nibble of input ...
East Croydon 20.79% interchange
a nibble of input ...
London Kings Cross 8.54% interchange
[trainee@easterton u114]$

]]